Global navigation satellite systems (GNSS) such as the Global Positioning System (GPS), the Compass navigation system, the Galileo positioning system and GLONASS all operate on the principle of trilateration based on signals received from satellites in each system. Each system generally functions by measuring some time shift in the signal received from each satellite. The time shift is a measure of the distance each signal traveled to reach the receiver. A computer in the receiver uses the known position of each satellite in orbit and the calculated distance from each satellite to determine the only location where the receiver could be located.
Measuring the time shift for each signal requires a high degree of precision; even a very small error can result in a calculated distance hundreds of meters from the receiver's actual location. Precise time shift measurements require the best possible signal from each satellite. Satellite signals are often degraded by interference. Interference may be caused by active signal jamming or simple noise such as from other electromagnetic devices or quick receiver antenna movement. GNSS signals are generally digital.
Interference may be reduced by spreading each signal over a wide frequency range, and by including filters to remove noise. Filters must be tuned for particular situations to be effective. A very narrow filter may provide a high quality signal, but may also filter out the signal completely in a noisy environment such as when the receiver is moving. A broad filter may provide a reasonable quality signal in a dynamic environment, but may not be able to filter out intense interference in a narrow frequency band.
Consequently, it would be advantageous if a method and apparatus existed that are suitable for increasing availability of a global navigation satellite system in a dynamic environment and in an environment of intense interference.